A controller that sets a norm of an output voltage vector of an inverter is proposed using a map that defines a relation between a required torque and a revolving speed for a three-phase electric motor equipped with a permanent magnet, and a norm of an output voltage vector of a voltage applying circuit (inverter) as disclosed in Japanese Patent Application Laid-Open Publication No. 2009-232531, for example.
Here, a phase of the output voltage of the inverter is defined by controlled variables of torque feedback control.
By the way, when an abnormality that is a decrease of magnetic flux in the permanent magnet equipped in the electric motor, or what is called demagnetization occurs, inconvenience such as a decrease of an actual torque relative to the required torque arises.
For this reason, a technology that determines the decrease of magnetic flux in the permanent magnet, or so-called existence of demagnetization is disclosed in Japanese Patent No. 4223880, for example.
Specifically, when using command voltage as the controlled variables for performing current feedback control so that command current becomes the required torque for three-phase electric motor, the existence of demagnetization is determined based on a difference of the command voltage and a standard value.
However, in a technology of defining the norm of an output voltage vector unambiguously according to a required torque disclosed in the above-mentioned Publication No. 2009-232531, it is difficult to determine the existence of demagnetization based on the norm of the output voltage vector.